The present invention relates to illumination optical systems and image projection apparatuses with the same, the illumination optical systems being suitable for liquid crystal projectors which project an enlarged image corresponding to an original image displayed on a liquid crystal panel (image display element) onto a screen, for example.
Conventionally, various image projection apparatuses (liquid crystal projectors) have been proposed, which project an enlarged image corresponding to an original image displayed on an image display element, such as a liquid crystal display element (liquid crystal panel), onto a screen.
Image projection apparatuses are well known, which use a reflective liquid crystal display element as the image display element (see Japanese Patent Laid-Open No. H02-250026). The image projection apparatus disclosed in Japanese Patent Laid-Open No. H02-250026 uses a polarization beam splitter as a means to introduce illumination light to the reflective liquid crystal display element and analyze light from the reflective liquid crystal display element.
Further, the image projection apparatus disclosed in Japanese Patent Laid-Open No. H02-250026 is provided with a quarter-phase plate as a means to prevent deterioration of contrast caused by a difference in inclination of polarized light in the polarization beam splitter, the quarter phase plate being located between the polarization beam splitter and the reflective liquid crystal display element.
In addition, color liquid crystal projectors are known, in which three image display elements (liquid crystal display elements) for R(red) light, G(green) light and B(blue) light are illuminated with respective color light components generated by color-separation of light from a light source (see Japanese Patent Laid-Open No.2000-305171). The color light components from the three image display elements are combined by a color-combining means and projected through a single projection lens on a screen or the like.
In recent years, image projection apparatuses need to project brighter images. Especially lately, illumination optical systems need to illuminate the reflective liquid crystal display element more brightly, compared to illumination optical systems (dark illumination optical systems) in which the incident angle of light onto the reflective liquid crystal display element is small (that is, its F-number is large (dark)) to emphasize contrast of projection images.
Therefore, it is necessary to increase the incident angle of light onto the reflective liquid crystal display element for achieving an illumination optical system (bright illumination optical system) capable of illuminating the reflective liquid crystal display element brightly and to improve contrast of projection images.
In Japanese Patent Laid-Open No. H02-250026, the phase plate utilizes a difference (phase difference) between the phase of light in the fast axis direction and the phase of light in the slow axis direction; the phase difference relates to the thickness of the phase plate. The thickness of the phase plate does not become a problem in terms of optical performance since the angle range in which light passes through the phase plate in the dark illumination optical systems is narrow.
However, in the bright illumination optical systems, since the distance (optical path length) through which light incident on the phase plate at the normal angle passes is different from the distance through which the light incident on the phase plate at a large angle passes, a phenomenon in which the phase differences are different in the same phase plate depending on the incident angles.
Consequently, correction of the inclination of the polarized light in the polarization beam splitter is not performed sufficiently, which deteriorates image contrast.